1. Field of the Invention
This invention relates to a picture encoding and/or decoding system, and more particularly is applicable to transmitting, recording, etc. a motion picture.
2. Description of the Prior Art
First of all, to compress picture information in transmitting picture data to remote places or recording it in a recording media such as a magnetic tape, there have been adapted various coding methods, such as predictive coding, transformation coding, and hybrid coding combining the predictive coding and the transformation coding together, which are methods comprising redundancy suppressing coding of picture signals.
Predictive coding comprises intrafield prediction and interframe prediction, the former being performed according to previous sample prediction and 1 line prediction utilizing correlations between adjacent pixels, while the latter being performed according to frame previous sample prediction and motion compensation (MC) prediction using interframe correlations.
The transformation coding is such that utilizing correlation picture signals have sampled values (hereinafter referred to as picture data) that are transformed to coordinates of which axis are orthogonal to each other, to thereby remove correlations between picture data for decreasing the amount of data. There are adapted orthogonal transformations in which so-called basic vectors are orthogonal to each other, where the pretransformation mean signal power is equal in total sum to mean signal power of so-called transformation coefficients obtained by orthogonal transformation, providing a power concentration degree to low frequency components which is excellent. Known examples are Hadamard transformation, Haar transformation, Karhunen-Loeve (K-L) transformation, discrete cosine transformation (DCT), discrete sine transformation (DST), slant transformation, etc..
The DCT divides a picture to picture blocks each consisting of n pixels in both horizontal and vertical directions (n .times. n) in spatial allocation, and orthogonal transforms picture data in picture blocks using cosine. DCT has a high speed computing algorithm, and is widely used in the transmission and recording of picture data since one chip LSI which enables real time transformation of picture data has been realized.
Moreover, DCT has substantially the same characteristic as the K-L transformation which achieves optimum transformation at a point of power concentration degree to low frequency components directly affecting efficiency. Therefore, it is possible to greatly reduce the amount of information by encoding only components to which power concentrates.
In transmitting and recording picture data, transformation coefficients which have been obtained by discrete cosine transforming the picture data are quantized as described, and then the picture data is subjected to variable length coding (for example, Huffman coding, run length coding, etc.) for further compression. The resulting coded data is added with synchronizing signals, parity, etc. for conducting transmission and recording.
However in the conventional method using the motion compensation (MC) hybrid DCT, motion regions are coded without questioning whether or not the motion regions are regions of reproduced pictures that are susceptible to degradation if the regions are judged motion regions after mode judgment of intrafield DCT/interframe DCT. Although there is no problem at a relatively high coding rate, the quantizing step of DCT coefficients becomes rough at a low coding rate, and there can be a case in which in reproduced picture degradation of visually important portions is susceptible to degradation.
In such a case, it is necessary to suppress the amount of information at portions where degradation is visually less important and increase the amount of information at visually important portions. It is considered that more effective compression can be realized by paying attention to the follow-up characteristic of the viewer's eyes to the speed of motion. However, it is not possible to obtain information which represents a degree of motion by adaptation using remainder energy according to the conventional methods.
Secondly, a conventional motion picture coding apparatus which codes television signals, for example, may consist of an apparatus which performs high efficiency coding according to a technique called discrete cosine transform (DCT), in which an amount of information is suppressed at portions where visual degradation is less important, with respect to the traceability of the viewer's eyes to follow the speed of motion.
In a conventional motion compensation coding technique, adaptation to the degree of motion is not conducted. For adapting to the degree of motion in addition to the motion compensation coding, there is an attempt which distinguishes between rapid motion and slow motion (including still region) in a picture.
Practically, classification is carried out according to the direction and absolute value of each motion vector, and then equal classes are found between adjacent blocks to make correlations for distinguishing fast motions from slow motions (including still regions) in the picture.
However, motion vectors are calculated about individual blocks. For this reason, motion vectors are discontinuous between blocks and an irregular class inevitably appears. As a result, there is a problem in that when only very small area in a relatively slow motion area, for example, is judged to have a fast motion, this very small area becomes susceptible to errors and is therefore detected as a degradation in picture quality.
Thirdly, typically, a high frequency speed exceeding 20 MHz is required in coding, certain motion picture signals, for example, high definition television signal (HDTV). To meet this requirement motion, picture signals of one picture plane are divided into several channels and are subjected to parallel processing thereby achieving sufficient processing speed.
Practically, in such a coding processing, to code motion picture signals at a high compression rate, there is adopted a high efficiency coding method of motion compression discrete cosine transformation (DCT) in which amount of information is suppressed at visually less important portions, with respect to the traceability follow-up capacity of the viewer's eyes to follow speed of motion.
Additionally, there is a technique in which as adaptation to the degree of motion is conducted, classification of motion is simultaneously conducted according to the direction and absolute value of motion vectors. In such a procedure, equal classes between adjacent blocks are subsequently detected and correlations thereof are made, so that fast motions and slow motions (including still regions) in a picture are distinguished.
However, in a motion picture coding method in which a picture is divided into a plurality of channels and classification is performed as well as motion compensation DCT, the border portions between channels search ranges of motion vectors become narrow as compared to normal ranges and thus, there may be a problem in that the motion vectors of these border portions do not have the same detectability or the motion vectors of portions other than the border portions. This can result in that directions and amounts of movements of motion vectors have no correlations with those adjacent blocks.
Furthermore, in motion adaptive quantization, channel border portions which have no vector correlation are presumed fast, irrespective of actual motions, and therefore quantization is roughened. As a result, there can be a problem in the case where a picture is divided into channels crossing the horizontal axis, for example, picture quality deterioration of vertical stripe patterns along borders of channels is produced in a reproduced picture.
In conducting motion compensation as described, vectors of border portions of channels are not provided with enough search ranges. Therefore, they become discontinuous with the surrounding vectors and in motion adaptive quantization, picture quality is degraded at the border of channels.